The present invention relates to a variable output voltage constant-frequency transmitter, and, in particular, to a transmitter capable of providing discrete output voltage level changes without sacrificing performance, simplicity and power supply efficiency.
Prior art signal transmitter devices in common use range from those defined by Class A to Class D operation. Class A transmitters, being continuous wave operation, have the greatest level of control on output voltage variability, but suffer from poor efficiency of operation and, therefore, are wasteful of power. Class D transmitters, on the other hand, hold out the greatest promise when considering efficiency, but generally lack sufficient output level control to be practical transmitters for satisfying the requirement for controllability over a range of output voltage levels. Methods known for controlling output voltage level changes include autotransformer coupling and power supply voltage variations. These techniques, however, contribute to the complexity and loss of efficiency and, therefore, the cost and desirability of the transmitter.
An application of transmitters of the type described herein is for telemetric communication between an external control device, such as a programmer, and a pacemaker inductor coil, which is included in an implanted pacemaker to be programmed. Typically, the programming information is in the form of modulated telemetered signals from the transmitter to the pacemaker. The type of information normally transmitted is known in the art and, therefore, will not be repeated here. The physiology of the patient having the pacemaker implanted determines, to a major degree, the output level required of the transmitter in order to effectively program the pacemaker.
Normally, transmitter output signals are sinusoidal in nature, with transmitter frequencies being established by inductor-capacitor tank circuits with its resonant frequency being a function of ##EQU1## In the above equation, L represents the value of the inductance, and C represents the value of the capacitance of the tank circuit.
Illustrative of the shortcomings of conventional methods of output level control, is, for example, an autotransformer-type transmitter, where inductance changes, due to selection of different transformer taps for voltage level changes, affect the resonant frequency of the transmitter output signal. Control of power supply output voltage, on the other hand, is found to be wasteful of power.
Accordingly, to accommodate required output level changes, while avoiding the degradation of efficiency or frequency shift due to inductance value changes, the present invention provides a controllable transmitter output voltage while maintaining highly efficient frequency stable operation.